1. Field of Invention
This invention is directed generally to printing duplex copy sheets from electronic page information, especially suitable for low cost electrostatographic, ink jet, ionographic or other on-demand page printers with an endless duplex paper path loop. More particularly, this invention relates to such printers which are integrated with on-line finishing devices.
2. Description of Related Art
The terminology "copiers", and "copies", as well as "printers" and "prints", is used alternatively herein. The terminology "imaging" and "marking" is used alternatively herein and refers to the entire process of putting an image, from a digital or analog source, onto paper. The image can then be Permanently fixed to the paper by fusing, drying, or other means. It will be appreciated that the invention may apply to any system in which the images are made electronically, including digital copiers.
Imaging systems, such as printers or copiers, typically include copy sheet paper paths through which copy sheets, such as plain paper sheets, that are to receive an image are conveyed and imaged. The process of inserting copy sheets into the copy sheet paper path and controlling the movement of the copy sheets through the paper path to receive an image on one or both sides is referred to as "scheduling". Copy sheets are printed by being passed through a copy sheet paper path that includes a marking station one or more times. Copy sheets which are printed on only one side, referred to as "simplex copy sheets", and in a single color usually pass through the copy sheet paper path a single time.
Multipass printing is used to print images on both sides of a copy sheet, referred to as "duplex printing", or to print a simplex sheet in multiple colors, where one pass is used for each color. There are two general modes in which copy sheets to be multipass printed can be scheduled: "burst mode" and "interleave mode".
When scheduling in "burst mode", copy sheets are inserted into, imaged, and output from the copy sheet paper path without any "skipped pitches" existing between each consecutive copy sheet. Various methods for scheduling copy sheets in "burst mode" are disclosed in, for example, the above incorporated U.S. Pat. No. 5,095,342.
A "pitch" is the portion (or length) of the copy sheet paper path in the process direction which is occupied by a copy sheet as it moves through the copy sheet paper path. A "skipped pitch" occurs when there is a space between two consecutively output copy sheets which is long enough to hold another copy sheet. Accordingly, when scheduling in "burst mode", copy sheets are output from the copy sheet paper path, and thus the imaging system, at a maximum rate because no skipped pitches exist between each consecutive copy sheet.
When scheduling copy sheets in "interleave mode", skipped pitches are provided between each consecutively scheduled copy sheet. That is, a space is provided between each copy sheet inserted into and output from the copy sheet paper path. While other copy sheets may be eventually inserted in the space between two consecutively input sheets, these other sheets are inserted at a later time and are thus "interleaved" with the previously inserted copy sheets.
The "interleave mode" of copy sheet scheduling is typically employed in imaging systems which are capable of duplex printing, i.e., forming images on both sides of a copy sheet. Many imaging systems which are capable of duplex printing include copy sheet paper paths in the shape of a loop. The scheduling process involves: a) inserting a copy sheet into the loop; b) forming an image on a first side of the copy sheet at an imaging station; c) inverting the copy sheet so that a second side of the copy sheet will face the imaging station when the copy sheet is reconveyed past the imaging station; d) forming an image on the second side of the copy sheet at the imaging station; and e) outputting the copy sheet from the paper path loop toward a final destination, such as a tray, a binder, finishing devices, stackers, etc.
One reason why the "interleave mode" of scheduling is frequently used when duplex printing relates to the manner in which the original images are provided to the imaging station. For example, when the imaging system is using a recirculating document handler (RDH) to cycle a simplex document over a platen for exposure to a light source for forming duplex copies of the document, the imaging system exposes every other sheet in the simplex document so that a duplex copy of the document can be formed. For example, all even numbered pages in the document are exposed first to form a copy set consisting of copy sheets having even numbered pages on one side. Then, the odd numbered pages in the document are exposed, and these odd numbered pages are formed on the second side of the copy sheets containing the even numbered pages on side one.
The set/offset mechanism in Xerox's 4135 Stacker module is designed to offset single sheet sets arriving at a rate of 135 ppm. With a speed of up to 180 ppm (7 pitch) on Xerox's Cyclone 4180 photocopier, the Stacker offsetting module cannot easily be extended to handle the faster rate of the Cyclone. In order to utilize the existing design and maximize productivity in the duplex mode, the interleave mode can be utilized.
The interleave mode allows completed duplex prints to exit the Input/Output Terminal with an empty pitch between them. This empty pitch provides time for the output device, e.g., the 4135 Stacker or a Signature Book Maker (SBM), to perform its function, for example, offsetting, binding, registration, etc. Moreover, depending on pitch mode and set size, there are circumstances or conditions in which no skipped pitch is provided between copy sheets from different sets at the Input/Output Terminal. Therefore, there is insufficient time for the output device to perform its functions.